Dry exhaust gas conditioning

Abstract: A method of treating exhaust gases, comprising exposing the gases to a first stage comprising silicon, or a silicon rich alloy or substance, and thereafter exposing the gases to a second stage comprising calcium oxide or a calcium oxide compound or calcium oxide containing material or soda lime containing material, characterised in that the first stage comprises particles of silicon, or a silicon rich alloy or substance which have been partially coated with copper.

Description: The purpose of the invention is to convert certain hazardous gaseous effluents by chemical reactions into safer solid or gaseous products. Preferably the chemical reagents used are solids and hence reduce the risk of carry-over contamination from the gas conditioning equipment. The solid chemical reagents used are easier and safer to handle than liquid reagents. The solid reagents lend themselves readily to encapsulation in exchangeable cartridges. The gaseous effluents can be the gases that come from etching of semi-conductor devices or from PECVD (Plasma Enhanced Chemical Vapour Deposition) on to semi-conductor materials; similar gases and vapours are used in several stages of manufacture in the semi-conductor manufacturing process. The effluent gases (and vapours) from reactive ion etching and plasma etching include chlorine, silicon tetrachloride, copper chloride (CuCl), aluminium chlorides (ie. AlCl 3 ), silicon tetrafluoride trifluoromethane (CHF 3 ), carbonyl fluoride (COF 2 ), carbonyl chloride (COCl 2 ), nitrogen trifluoride (NF 3 ), boron trichloride (BCl 3 ), boron tribromide (BBr 3 ), hydrogen chloride (HCl), carbon tetra-chloride (CCl 4 ), chloro-fluoro carbon gases and others. Additional gases and vapours that are sometimes found in the effluent of PECVD operations include silane (SiH 4 ), dichlorosilane (SiCl 2 H 2 ), tetraethylorthosilicate (Si(OC2 2 H 5 ) 4 ), diborane (B 2 H 6 ), trimethyl borate (B(OCH 3 ) 3 ), phosphine (PH 3 ), trimethylphosphite (P(OCH 3 ) 3 ), arsine (AsH 3 ) and others. The current practice is to allow these gases/vapours to enter an exhaust duct to be transported to a wet scrubber positioned at the exhaust end of the lines. This method is occasionally supplemented by local placement of scrubbers. For etching these may be either:- (a) Of the activated charcoal type to trap the acid gases. This method can trap up to 15% of its charcoal weight of Cl 2 . The product is charcoal with the effluent gases trapped but not converted to safer products and some of these effluent gases will be liberated if the charcoal is burnt. (b) Of the wet type; there are several designs of wet scrubber, most using either a Venturi stage or a packed tower or a combination of both. These designs have been used for larger applications: when placed local to the source of the contamination they present the danger of contamination of the process chamber with their aqueous solutions of caustic type media. The contamination can result from a back streaming type of mechanism or from a massive suckback. (c) The chemical filter type utilising a combination of adsorption and chemical adsorption. These are expensive and do not have a good capacity per unit volume. PECVD scrubbers can be as above but often have a Burn box gas conditioning unit introduced locally. These units burn the effluent gases to form their oxides and water vapour. They suffer from blockage problems and there are normally large volumes of oxides to contend with. Chemical exhaust gas conditioning unit operating in two main sections and an optional third section were described in our prior WO 89/11905 A1 assigned to the present Applicants. In these units, the effluent gases pass first through a section in which the active chemical components is elemental silicon. Second, through a section in which the active component is lime, or soda lime (calcium oxide or calcium hydroxide). Third, and optionally, through a section in which the active components are lime and copper oxides (CuO or Cu 2 O). These sections can be housed in a single container (Figure 1) or compartmentalised in different containers (Figure 2) so that the gases can flow from one to the next. Alternatively the sections can be combined, the silicon mixed with the lime/soda lime in a single reactor. Broadly stated, the invention of our prior Application consisted of apparatus for treatment of exhaust gases, comprising means for exposing the gases to silicon, or a silicon rich alloy or substance, and means for then exposing the gases to calcium oxide or a calcium oxide compound or calcium oxide containing material, or a soda lime containing medium. A preferred embodiment of the prior Application comprised a first (silicon) stage, and a second (calcium oxide) stage arranged in sequence and conveniently a single gas treatment chamber having in series silicon and calcium oxide containing zones. In the first stage, it was stated that the silicon should have a purity greater than 90% but preferably greater than 97%. It could be mixed with "inert" substances such as silica or alumina or even with lime (CaO) or magnesia (MgO). It could also, though with increased problems be alloyed with any of the metals Cr, Mn, Fe, Co or Ni, of which ferrosilicon or manganesesilicon are cheap examples. Silicon carbide, which is significantly less reactive but behaves more slowly in a similar way to silicon, was also possible. This silicon stage might be heated to a temperature above 200°C and preferably between 350°C and 550°C. In an alternative embodiment of our prior Application, it was stated that copper of a copper rich material may be added to the first stage to promote the scrubbing of nitrogen trifluoride (NF 3 ) gas from nitrogen rich exhaust streams. The copper promotes the formation of N 2 F 4 which will react with silicon to form silicon fluoride (SiF 4 ) which in turn reacts with the lime second stage. The present invention is concerned with a method (and apparatus) for the treatment of exhaust gases, which includes the presence of copper in the first stage in a particular form. In accordance with the invention there is provided a method of treating exhaust gases which include nitrogen trifluoride, the method comprising (a) exposing the exhaust gases to a first stage of particles of silicon, silicon carbide or silicon alloyed with any of the metals chromium, manganese, iron, cobalt and/or nickel; and then (b) exposing the exhaust gases to a second stage of calcium oxide, a calcium oxide compound, calcium oxide containing material, soda lime or soda lime containing material, the soda lime comprising calcium hydroxide with sodium hydroxide or calcium hydroxide with potassium hydroxide; and wherein the particles of silicon, silicon carbide or silicon alloyed with any of the metals chromium, manganese, iron, cobalt and/or nickel, are partially coated with elemental copper. In particular, it is advantageous that i) the silicon is not completely coated with the copper to ensure that the copper does not completely cover, and thereby passivate, the silicon particles in that both the silicon and the copper play an important role in the reactions in which the exhaust gases, and NF 3 in particular, are eliminated from the gas stream as it passes through the exhaust gas conditioning unit ii) the silicon and the copper are in intimate physical contact over at least part of the surface areas of the respective particles. Although the presence of copper in the first stage in accordance with the invention has been found to be particularly useful for the treatment of exhaust gas streams (especially nitrogen rich streams) containing NF 3 , it can also be useful in the scrubbing of other exhaust gases, for example those used in the etching of semi conductor devices or in the plasma enhanced chemical vapour deposition on to semi conductor devices. Preferably the silicon is coated with the copper by coating the silicon (or silicon rich alloy or substance) particles with a solution of a copper compound which can subsequently be decomposed (or otherwise reacted) to form elemental copper on the surface of the particles preferably in an inert or a reducing atmosphere, thereby preventing the formation of copper oxide. The copper compound can be, for example, copper oxalate contained in a suitable solution, for example, a lactose solution, in which the silicon particles can be immersed and coated with the solution; the oxalate can then be decomposed by heating at say 300 - 500°C, preferably in a nitrogen (or other inert or reducing atmosphere) atmosphere. With regard to the reaction mechanisms, it is known that NF 3 reacts readily with copper to form nitrogen fluoride (N 2 F 4 ) but that the formation of copper fluoride (CuF 2 ) inhibits this reaction. This aspect of the invention is based on the surprising discovery that partial coating of the silicon particles with copper would allow the NF 3 /Cu reaction to proceed as above but thereafter to allow a) the resulting copper fluoride to react with the silicon to produce elemental copper and silicon fluoride (SiF 4 ) and b) the resulting nitrogen fluoride (N 2 F 4 ) - a much more reactive gas than nitrogen trifluoride (NF 3 ) - to react readily with silicon, even at relatively lower temperatures. In the second stage, the calcium oxide may be in the form of lime. This can be CaO or CaO mixed with "inert" materials such as graphite (or coke), limestone (CaCO 3 ) or magnesia (MgCO 3 ). It could also be slaked lime (Ca(OH) 2 ), which would give a more fuming product with SiCl 4 but would be fine for hydrides (ie. silane diborane phosphone or arsine), or even CaCO 3 , kept at a slightly higher temperature so that it is beginning to decompose to CaO, to dolomite CaCO 3 MgCO 3 . The temperature of this stage may be above 100°C and preferably between 350°C and 600°C. Further purification may be provided in a third stage using copper oxide (CuO or Cu 2 O) as the active ingredient, and this may be supported on calcium silicate or (calcium oxide plus calcium silicate). The invention may be performed in various ways and one specific embodiment with a number of modifications will now be described by way of examples with reference to the accompanying drawings, in which:- Figure 1 is a diagrammatic sectional view through one form of exhaust gas conditioning unit or reactor column according to the invention; Figure 2 illustrates the withdrawal of the cartridge unit of the reactor column of Figure 1; Figure 3 is a diagrammatic illustration of a piping system including the reactor column of Figure 1; and Figure 4 is a diagrammatic illustration of a dual conditioning system. In the illustrated example of Figure 1 the unit comprises a common vertical cylinder or column 10, which is in the form of a removable cartridge, having an exhaust gas inlet 11 at the lower end and an outlet 12 at the top. The column is surrounded by an electrically heated furnace 13 and there may be a spaced control thermo-couples 14 at different levels. The column is divided into three sections within the same common cylinder. The bottom section 16 contains silicon partially coated with copper, the centre section 17 contains lime, and the top section 18 contains copper oxide. The First Stage (Silicon/copper) The effluent gases first pass through a bed of granular silicon which has been partially coated with copper in a manner described below at an elevated temperature. The silicon used is most economically a metallurgical grade of crystalline or poly-crystalline material with a purity of approximately 98.5%. Higher purity silicon can be used effectively. It is also possible to use silicon-iron alloys known as "ferrosilicon" in place of or mixed with silicon: this increases the ability of the system to retain boron, phosphorous or arsenic, but gas transport of slightly volatile iron halides from the silicon section can cause blocking of gas pipes. The silicon used should be in the form of granules or lumps, graded in size depending on the expected gas flow through the system. Lumps passing a 100 mm mesh sieve and retained by a 20 mm mesh sieve are ideal for giving very low impedance with high gas velocities, whereas granules, say passing a 15 mm mesh sieve and retained by a 3 mm mesh sieve also work, but with a higher flow impedance. Powdered silicon is undesirable as it may catch fire in an oxygen rich effluent stream. The silicon copper-coated should be heated in the temperature range 200 - 700°C with the preferred temperature 350 - 550°C. The silicon can be contained in any suitable container, eg. stainless steel, mild steel, graphite, ceramic or quartz, and heat supplied through the walls as shown in the drawings, or alternatively by induction heating or by an internally placed heat source. The Second Stage (Lime/Soda Lime) The hot gases/vapours emerging from silicon stage then pass through a bed of hot granulated lime. This converts silicon tetrahalides into a calcium silicate and a calcium dihalide in an exothermic reaction. It also converts boron trihalides into calcium borate and calcium dihalides and decomposes tetraethylorthosilicate vapour into calcium silicate and diethyl ether, and residual silane or borane are converted into a mixture of hydrogen and calcium silicate and silicon or calcium borate and boron. The lime needs to be soft permeable structure strong enough to support the weight of the column without crushing to powder. The lime must be kept free of water vapour during storage and may be of a similar size to that described in the silicon section above. The lime section needs to be maintained at an elevated temperature. A temperature in the region of 100 - 600°C would be suitable with a preferred range of 250 - 550°C. In order to promote the maximum utilisation of the lime lumps it is desirable occasionally to cycle the lime temperature widely in order to promote cracking of the surface and the subsequent exposure of the underlying layers. A temperature fluctuation from 100°C to 400°C is suitable to promote this thermal cracking. (Optional) The Third Stage (Copper oxide, silica and lime) This stage will be required for some PECVD applications or for applications having similar exhaust gas emissions. The gases and vapours emerging from the second stage will enter the third stage. The effluent gases/vapours present are residual phosphine arsine, elemental phosphorous and arsenic. These substances react with the copper oxide rich reagent present to form copper phosphate or arsenate and water vapour. In addition carbon monoxide is converted to carbon dioxide and hydrogen will reduce the copper oxide to copper and water. The reagent (a mixture of copper oxide, silica and lime) must first be prepared in a suitable form to be loaded into the reaction vessel. The requirement is to provide a pellet that is porous, of a size to allow the free passage of gas, and strong enough to maintain its integrity within the column. The copper oxide rich section is maintained at an elevated temperature within the range 150 - 600°C with an optimum temperature range of 200 - 400°C. Figure 3 illustrates a possible piping system allowing by-passing of the column 10 during cartridge replacement. This is achieved by by-pass pipe 30, by-pass valve 31 and column isolation valves 32,33. Valve 31 may operate automatically in the event of a significant pressure build up purge and sampling ports 34,35 are provided. Figure 4 illustrates a dual system in which there are two separate silicon/copper stages 20, 21 positioned in parallel and two separate lime stages 22,23 also in parallel. Valves 24 are provided to control the flow in and out of the different stages and there is also a cross-communicating passage 26 controlled by valves 27 which allow gases from one side of the system to be transferred to the other side. This may be useful in a system in which the silicon/copper and calcium oxide stages are separate, allowing for a continuous flow to be maintained. Tests were conducted on reactor columns having copper additions to the silicon first stage in which the separation of NF 3 from nitrogen streams in particular were recorded. The reactor column used in these tests in particular comprised an outer stainless steel casing having a first stage of silicon/copper prepared as described below and an upper second stage of lime, each stage being substantially equal in size within the casing. Means were provided to pass a stream of nitrogen through the reactor column from a nitrogen cylinder and via a pressure regulator to the base of the reactor column. Separate means were provided to pass a quantity of NF 3 gas from a cylinder and via a pressure regulator and a mass flow controller and into the nitrogen stream at the base of the reactor. The reactor itself was surrounded by a hollow cylindrical heater incorporating electrical resistance heater means and the area inside the reactor column 1 at the interface between the first and second stages was equipped firstly with a thermometer in the form of a thermocouple and secondly with a probe of a mass spectrometer. The reactor column silicon/copper first stage was formed by adding copper oxalate to a solution of lactose to form a creamy paste. Silicon granules were mixed with the paste and stirred to coat the silicon with the paste and the mixture left to dry. The coated silicon was thereafter loaded into the lower half of the reactor column and heated at 500°C for about 8 to 12 hours in a nitrogen atmosphere. This resulted in a partial coating of the silicon granules with elemental copper. The reactor column lime second stage was then added within the stainless steel outer coating. In use of the column, nitrogen at a flow rate of 100 standard l/hr was passed through the column for several hours to stabilise the system. The mass spectrometer was set to monitor, inter alia, the NF 3 + /NF 2 + species to establish whether NF 3 remained in the column at the top of the silicon/copper first stage. Various tests were then conducted by passing a N 2 /NF 3 gas mixture through the column at the rate of 25 standard cm 3 /min NF 3 in 1.6 standard 1/min N 2 at different temperatures including ambient temperature. These tests showed that NF 3 (by virtue of the presence or otherwise of NF 3 + /NF 2 + peaks) was present at ambient temperatures but that at temperatures above 200°C its presence was substantially reduced. Preferably the operating temperature of the first stage of the reactor column exceeds 230°C which generally was shown to provide a 99.9% removal of NF 3 from the nitrogen stream. Most preferably the operating temperature of the first stage is at least 375°C, more advantageously 400°C or more, for all traces of NF 3 to be removed from the reactor column. As will be known soda lime is normally calcium hydroxide with sodium hydroxide. Reference in this specification to soda lime should also include a mixture of calcium hydroxide and potassium hydroxide.

Claims: A method of treating exhaust gases which include nitrogen trifluoride, the method comprising (a) exposing the exhaust gases to a first stage of particles of silicon, silicon carbide or silicon alloyed with any of the metals chromium, manganese, iron, cobalt and/or nickel; and then (b) exposing the exhaust gases to a second stage of calcium oxide, a calcium oxide compound, calcium oxide containing material, soda lime or soda lime containing material, the soda lime comprising calcium hydroxide with sodium hydroxide or calcium hydroxide with potassium hydroxide; and wherein the particles of silicon, silicon carbide or silicon alloyed with any of the metals chromium, manganese, iron, cobalt and/or nickel, are partially coated with elemental copper. A method according to Claim 1 in which the exhaust gases comprise a nitrogen rich gas stream containing nitrogen trifluoride (NF 3 ). A method according to any preceding claim in which the silicon or silicon rich alloy or substance has been coated with the copper by coating the silicon or silicon rich alloy or substance with a solution of a copper compound which has then subsequently been decomposed or otherwise reacted to form elemental copper on the surface of the particles. A method according to Claim 3 in which the decomposition or other reaction is performed in an inert or reducing atmosphere. A method according to Claim 3 or Claim 4 in which the copper compound is copper oxalate. A method according to Claim 5 in which the copper oxalate is in a lactose solution. A method according to any preceding claim in which the exhaust gases are subsequently exposed to a third stage comprising copper oxide or a copper oxide rich reagent.

Inventors: SMITH JAMES R, TIMMS PETER L, SMITH, JAMES R., TIMMS, PETER L.
Applicants: BRITISH OXYGEN CO LTD, BOC LIMITED
Application year: 1994
Main classification: B01D53
IPC codes: B01D53